1. Field of the Invention
The present invention generally relates to an image reading apparatus mounted on a copy machine, a printer, a facsimile machine, a scanner, etc., and a method of accelerating the drive of a reading portion of an image reading apparatus.
2. Description of the Related Art
With a conventional image reading apparatus for reading an image by scanning an optical unit (e.g. lamp, mirror) across an original document or the like, the optical unit accelerates while moving from a stop position (home position) to an image reading position to a predetermined speed, and then begins to read in a constant speed from the image reading position. However, in a case where a motor of the image reading apparatus is rapidly accelerated or decelerated, the optical unit cannot switch smoothly to the constant speed due to inertia caused by the rapid acceleration or deceleration thereof, thereby causing the optical unit to overshoot. Therefore, in a case where the optical unit reaches the image reading position before the overshot is recovered, the optical unit cannot move at a constant speed, thereby creating irregular images such as shaky images, or overlapped images. Particularly, in reading with a full color reading apparatus employing an RGB (Red-Green-Blue) tri-linear CCD (Charge Coupled Device) sensor, the optical unit lurches to cause the RGB tri-linear CCD sensor to deviate slightly from an RGB reading position, thereby resulting to problems such as inadvertently coloring both sides of a black line. An excessive rapid acceleration being uncontrollable for a torque of the motor causes the motor to lose synchronism, and result to stoppage of the image reading apparatus. However, attempting to solve the foregoing problem by reducing rapid acceleration requires a large runway space for acceleration, thereby requiring a large sized apparatus which results to an increase in manufacture cost. Therefore, in order to provide an image reading apparatus of high speed, small size, and low cost for enhancing productivity, the image reading apparatus is required to accelerate to high speed while restraining the shock or overshoot caused when the rate of acceleration is changed.
For example, in a conventional art shown in Japanese laid open publication No. Hei10-23215 for restraining overshoot, a motor is accelerated to a target rate from the exact point where the motor reaches peak speed in response to a rapid increase in the driving rate. Nevertheless, in a case where magnification for image reading is adjusted by changing the scanning speed of the optical unit, acceleration needs to achieve a suitable speed for any given magnification factor within the range of the magnification. Therefore, CPU for control would be burdened with a considerable load since an acceleration curve in achieving a required speed is to be calculated each time and since an acceleration rate is to be controlled for achieving smooth acceleration.
For solving such problem, another conventional art shown in Japanese patent laid open publication No. Hei8-186690 provides a memory apparatus storing therein curves for acceleration and deceleration beforehand, and controlling acceleration and deceleration by reading the curves from the memory apparatus.
Nevertheless, since the conventional art shown in Japanese patent laid open publication No. Hei10-23215 is not mainly aimed to restrain the problem of overshoot, the degree of overshoot and the timing of overshoot for the conventional art largely depend on factors such as the property of the driving components or the assembling method thereof. Accordingly, with the conventional art, the point at which peak speed is observed would be inconstant, and the motor may lose synchronism by failing to switch acceleration rate at the exact timing.
In a case of the conventional art shown in Japanese patent laid open publication No. Hei8-186690, an increase in acceleration rate is required for shortening runway length if a single table for acceleration is used to cover all ranges of reading speed. Such case causes problems such as irregular images and loss of synchronism of the motor. An attempt may be made to solve the foregoing problems by preparing a plurality of tables suitable for all reading speeds. For example, in a case of changing magnification from 25 percent to 400 percent, the fastest reading speed would be 16 times faster than the slowest reading speed. A considerably large memory capacity for storing the tables is thus required for covering such speed range.
It is therefore an object of this invention to provide an image reading apparatus, having high speed, small size and low cost, capable of reducing CPU load and capacity required for storing a table therein.
It is a general object of the present invention to provide an image reading apparatus that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by an image reading apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an image forming apparatus, including a reading portion for reading an image by scanning an original document, driving means for driving the reading portion, and memory means for storing therein a plurality of driving frequencies for enabling the driving means to switch to a variety of speeds and drive in a step by step manner, and for storing therein a table indicative of a switching point for switching one driving frequency of the plurality of driving frequencies to another driving frequency of the plurality of driving frequencies, wherein one driving frequency of the plurality of driving frequencies is switched to another driving frequency of the plurality of driving frequencies at the switching point.
The reading portion of the image reading apparatus comprising a lamp, a mirror, and a carriage for supporting the lamp and the mirror, is formed as a single unit. The reading portion having a substantial amount of weight gains speed due to moment of inertia created when the reading portion is driven. The reading portion having gained speed will therefore overshoot when slowed down. The degree of overshoot increases in association with the increase in the difference between acceleration rate and deceleration rate, and also in association with the increase in the moment of inertia. A method of forming a reading portion as light as possible or a method of reducing the driving rate could be introduced for reducing overshoot. Nevertheless, there are limits in lightening the weight of the reading portion, and the reduction of driving rate would lower the performance of the image reading apparatus. In a conventional method, a reading portion before reaching an image reading area is moved to a prescribed point as fast as possible, and is then slowed down to a speed for preventing an overshoot upon reaching the proximity of the image reading area. The conventional method, however, has a drawback of requiring a considerable amount of memory capacity for storing therein various curves to cover all reading speeds corresponding to changes in magnification. In solving the foregoing problem, this invention uses a table indicative of a point for switching one driving frequency of a plurality of driving frequencies to another driving frequency of a plurality of driving frequencies, and determines driving frequency with reference to the table.
With the present invention, a memory means includes a table indicative of a switching point for switching one driving frequency of the plurality of driving frequencies to another driving frequency of the plurality of driving frequencies, in which one driving frequency of the plurality of driving frequencies is switched to another driving frequency of the plurality of driving frequencies at the switching point, thereby providing consistent prevention of an overshoot with minimal memory capacity.
Further, in the image reading apparatus of this invention, the plurality of driving frequencies include a low acceleration driving frequency and a high acceleration driving frequency, wherein the high acceleration driving frequency is switched to the low acceleration driving frequency at the switching point.
The plural driving frequencies are basically formed by combining 2 types of frequencies. That is, the driving means is first driven with the high acceleration driving frequency, then, the high acceleration driving frequency is switched to the low acceleration driving frequency upon reaching the switching point, and then the low acceleration driving frequency is switched to a constant reading speed upon reaching a prescribed reading speed selected (determined) from a control panel or the like. Accordingly, gradual acceleration can be provided consistently with simple control with a controller (e.g. CPU) by switching the high acceleration driving frequency to the low acceleration driving frequency at the switching point.
Further, in the image reading apparatus of this invention, the switching point is determined as a point where the driving means reaches a prescribed driving rate, or as a prescribed elapsed time from an initial drive of the driving means.
The switching point is a point where the driving means reaches a prescribed driving rate after being started and gradually accelerated. Since speed and time are closely related, the switching point may also be determined as a prescribed time elapsed from the start of the driving means. Accordingly, a precise point for switching the driving rate can be determined since the switching point is defined as a point where a driving rate of the driving means reaches a prescribed rate, or as a prescribed elapsed time from the initial drive of the driving means.
Further, in the image reading apparatus of this invention, each driving frequency of the plurality of driving frequencies is predetermined according to a reading speed selected (determined) with a control panel or the like, wherein each driving frequency of the plurality of driving frequencies is predetermined to enable the driving rate of the driving means to accelerate to the reading speed before the reading portion reaches an image reading area after the high acceleration driving frequency is switched to the low acceleration driving frequency at the switching point, wherein each driving frequency of the plurality of driving frequencies is predetermined to prevent the driving means from causing an overshoot.
Each driving frequency of the plurality of driving frequencies is predetermined according to a reading speed of the image reading apparatus. It is to be noted that each of the driving frequencies is required to be predetermined to enable the driving speed of the driving means to be accelerated to the reading speed before the reading portion reaches an image reading area after the driving frequency is switched from the high acceleration driving frequency to the low acceleration driving frequency at the switching point. In addition, each of the driving frequencies is required to be predetermined to prevent the driving means from causing an overshoot. Therefore, only a minimal runway length will be required and overshoot can be restrained to a smallest degree since each of the driving frequencies is predetermined for enabling the driving speed of the driving means to accelerate to the reading speed before the reading portion reaches an image reading area after the driving frequency is switched from the high acceleration driving frequency to the low acceleration driving frequency at the switching point, and for preventing the driving means from causing an overshoot.
Further, in another image reading apparatus of this invention, the image reading apparatus may include a calculating means for calculating a driving speed of the driving means in correspondence to the reading speed, wherein the switching point is determined according to the result of the calculating means.
In the aforementioned case, driving frequency is predetermined according to the reading speed of an image reading apparatus with use of a table. This case however is not to be restricted to such particular image reading apparatus. This case requiring no preparation of tables includes a calculating means for calculating driving rate according to reading speed selected (determined) from a control panel or the like, in which the calculating means calculates driving speed each time for determining the switching point. Since the switching point is determined according to the result of the calculating means, the preparation of a table is not required, thereby providing versatility for various types of image reading apparatuses.
Further, in the image reading apparatus of this invention, the driving means is decelerated in an opposite order with respect to acceleration of the driving means by referring to the switching point stored in the table.
The table used for gradual acceleration has a point for switching driving frequency stored therein. The gradual acceleration is performed in the following order: driving in a high rate, switching driving frequency at a switching point, driving in a low rate, driving at reading speed. In an opposite order with respect to the gradual acceleration, a gradual deceleration is performed in the following order: driving at reading speed, driving at a low rate, switching driving frequency at the switching point, driving in a high speed. Since the table having the switching point included therein can also be used for deceleration by driving the driving means in an opposite order with respect to acceleration, the required number of tables would be minimal, thereby saving memory space for the memory means such as RAM (Random Access Memory) or the like.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.